Aerospace and Electronic Systems Magazine March 2018 - 4

Feature Article:

DOI. No. 10.1109/MAES.2018.170027

Conceptual Study on Bistatic Shipborne High Frequency
Surface Wave Radar
Hongbo Sun, Nanyang Technological University, Singapore

INTRODUCTION
High frequency surface wave radar (HFSWR) operates in a high
frequency (HF) band (3-30 MHz) within which the surface wave
propagation mode allows beyond-the-horizon detection of targets,
as well as remote sensing of ocean parameters. Specifically, the
vertically polarized electromagnetic waves at the lower end of the
HF band (3-12 MHz) experience relative low losses when propagating over ocean water that enable the HFSWR to capture the
echo signals from the sea surface, ships, and low-flying aircrafts
far beyond the horizon.
The history of HF radar stretches back to the early warning radar systems developed during World War II, e.g., the famous Chain
Home system built by the United Kingdom in 1938 to warn against
German bomber attacks. However, during that decade, the radar
engineers had almost no knowledge of surface wave propagation
and the interactions between the electromagnetic signal and the sea
wave. In 1955, Crombie [1] observed the strong resonance effect
between the electromagnetic signal and the sea waves with certain
wavelengths, i.e., the discrete frequency shifts of the sea echo above
and below the carrier produced by sea waves approaching and receding from the radar. This phenomenon is called Bragg scatter. In
the 1970s, Barrick [2] and Barrick et al. [3] analyzed and deduced
the physical mechanism of the first-order and high-order Bragg
scatters for the HF band, respectively. These pioneering works
greatly stimulate interest and development of HFSWR systems. In
the past 30 years, many coast-based HFSWR systems have been
successfully developed and applied in the research or operational
missions of oceanography remote sensing and ship and aircraft target detection beyond the line of sight. The HFSWR has been widely
used by many countries for maritime surveillance in exclusive economic zones (EEZs) up to 200 nautical miles from the coast [4]-[6].
Typically, the conventional HFSWR system consists of a transmitting antenna and a linear receiving antenna array, which are col-

Author's current address: Temasek Laboratories, Nanyang
Technological University, Research TechnoPlaza, 9 Storey,
BroderX Block, 50 Nanyang Drive, Singapore, 637553 Singapore, E-mail: (ehbsun@ntu.edu.sg).
Manuscript received January 29, 2017, revised July 8, 2017,
September 19, 2017, and ready for publication October 29,
2017.
Review handled by D. O'Hagan.
0885/8985/18/$26.00 © 2018 IEEE
4

located near the coastline (Figure 1). The transmitting antenna is
usually a broadband antenna with broad beam in azimuth to cover
a wide area of interest. The receiving array is usually formed by a
number of antenna elements (e.g., 16 elements) with a total length
of a few hundred meters to achieve the desired directive gain and
azimuth angular accuracy and resolution. The key aspects of the
HFSWR system for maritime surveillance, such as surface wave
propagation, environmental noise, sea clutter, ionospheric clutter,
external cochannel interferences, and associated radar subsystem
design requirements, were addressed in [4]-[6]. The fundamental
processing techniques for ship detection and tracking were summarized in [7]. The characteristics of various interferences or clutters encountered in HFSWR, as well as the corresponding adaptive
beamforming schemes, were presented in [8]. Readers can refer to
these references for more details about the specific phenomenon or
problems in HFSWR, as well as the technical solutions.
Although conventional shore-located HFSWR has demonstrated superior performance as an efficient and persistent sensor
system for maritime surveillance, its detection range still cannot
be extended to the open sea far from the coastline. One intuitive solution to solve this problem is deploying the HFSWR on
a ship platform. Benefiting from the advantages of a mobile and
relocatable platform, the shipborne HFSWR can freely moves in
the area of interest. Compared to shipborne microwave radars,
the shipborne HFSWR can overcome the line-of-sight limitation
and achieve a longer detection range. An experimental monostatic shipborne HFSWR demonstrator was reported in [9], and
the measured sea clutter characteristics were compared with
theoretical analyses. It was shown that because of the motion of
the ship platform, the sea clutter energy spreads in Doppler and
no longer concentrate at Bragg frequencies. Thus, the radar detection performance will be significantly degraded if the target's
Doppler falls within the spreading sea clutter region. In principle,
the space-time adaptive processing (STAP) technique can be applied to suppress the spreading sea clutter; this has been validated
in simulations [10] and experimental trials [11] for monostatic
shipborne HFSWR.
Because of the bulky size of the receiving antenna array,
monostatic HFSWR can only be installed on large ships, which
greatly restricts its applications. To solve this problem, an innovative bistatic HFSWR concept was proposed in [12], which consists of a long transmitting antenna array on the coast and single
receiving antenna on the ship. The transmitting array transmits
orthogonal waveforms, and the digital beamforming can be

IEEE A&E SYSTEMS MAGAZINE

MARCH 2018



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